Call- by- Reference Func0ons Procedural Abstrac0ons Numerical Conversions CS 16: Solving Problems with Computers I Lecture #9

Transcription

1 Call- by- Reference Func0ons Procedural Abstrac0ons Numerical Conversions CS 16: Solving Problems with Computers I Lecture #9 Ziad Matni Dept. of Computer Science, UCSB

2 Announcements Homework #8 due today Lab #5 is due on Friday at Noon Homework Solu>ons are now online at: hap://cs.ucsb.edu/~zmatni/cs16/hwsolu>ons/ Your grades are NOT ON GAUCHOSPACE anymore. Instead go to: hap://cs.ucsb.edu/~zmatni/cs16/cs16grades_fa2016.htm 10/25/16 Matni, CS16, Fa16 2

3 Lecture Outline Call- By- Reference Parameters Using Procedural Abstrac>on Binary Arithme>c 1: Conversions 10/25/16 Matni, CS16, Fa16 3

4 Call- by- Value vs Call- by- Reference When you call a func>on, your arguments are ge]ng passed on as values At least, with what we ve seen so far The call func(a, b) passes on the values of a and b You can also call a func>on with your arguments used as references to the actual variable loca>on in memory Why would we want to do that? 10/25/16 Matni, CS16, Fa16 4

5 Call- by- Reference Parameters Call-by-reference parameters allow us to change the variable used in the function call Arguments for call-by-reference parameters must be variables, not numbers i.e. fn(var), not fn(5) We use the ampersand symbol (&) to distinguish a variable as being called-by-reference, in a function definition 10/25/16 Matni, CS16, Fa16 5

6 Call- by- Reference Parameters Recall that, up until now, we have changed the values of formal parameters in a function body, but we have not changed the arguments found in the function call! Example: when you call func(a, b, c), you might get a returned value for func, but a, b, and c do not change after the call. So if you want to get an input (via cin) using a func>on using call- by- value, it wouldn t work! 10/25/16 Matni, CS16, Fa16 6

7 Call- by- Reference Example void get_input(double& f_variable) { using namespace std; cout << Convert a Fahrenheit temperature << to Celsius.\n << Enter a temperature in Fahrenheit: ; cin >> f_variable; } & symbol (ampersand) identifies f_variable as a call-by-reference parameter Has to be used in both declaration and definition! 10/25/16 Matni, CS16, Fa16 7

8 Call- By- Reference Details Call-by-reference works almost as if the argument variable itself is substituted for the formal parameter, not the argument s value In reality, it s the memory location of the argument variable that is given to the formal parameter Whatever is done to a formal parameter in the function body, is actually done to the value at the memory location of the argument variable 10/25/16 Matni, CS16, Fa16 8

9 Call- by- Reference Behavior Assume variables first and second are assigned memory addresses 1010 and 1012 by the compiler, respec>vely Now a func>on call executes: get_numbers(first, second); The func>on is defined as: void get_numbers(int& first, int& second) { cout << Enter two integers: cin >> first >> second; } The func>on may as well say: void get_numbers(the int variable at memory location 1010, the int variable at memory location 1012) { cout << Enter two integers: cin >> the variable at memory location 1010; >> the variable at memory location 1012; } 10/25/16 Matni, CS16, Fa16 9

10 Call- by- Reference Behavior Ajer the func>on is called, the values of first and second are changed not just in the func>on, but in actual memory The values of first and second are in the calling block 10/25/16 Matni, CS16, Fa16 10

11 Call by- Reference vs by- Value Call-by-reference The function call: f(age); Memory Name Location Contents Call-by-value The function call: f(age); age initial 1002 A hours void f(int& ref_par); void f(int var_par); 10/25/16 Matni, CS16, Fa16 11

12 Example: swap_values void swap(int& variable1, int& variable2) { int temp = variable1; variable1 = variable2; variable2 = temp; } If called with swap(first_num, second_num); first_num is substituted for variable1 in the parameter list second_num is substituted for variable2 in the parameter list temp is assigned the value of variable1 (first_num) since the next line will loose the value in first_num variable1 (first_num) is assigned the value in variable2 (second_num) variable2 (second_num) is assigned the original value of variable1 (first_num) which was stored in temp 10/25/16 Matni, CS16, Fa16 12

13 Mixed Parameter Lists Call-by-value and call-by-reference parameters can be mixed in the same function Example: void good_stuff(int& par1, int par2, double& par3); par1 and par3 are call-by-reference formal parameters Changes in par1 and par3 change the argument variable par2 is a call-by-value formal parameter Changes in par2 do not change the argument variable 10/25/16 Matni, CS16, Fa16 13

14 Choosing Parameter Types How do you decide whether a call-by-reference or call-by-value formal parameter is needed? Does the function need to change the value of the variable used as an argument? Yes? Use a call-by-reference formal parameter No? Use a call-by-value formal parameter 10/25/16 Matni, CS16, Fa16 14

15 Cau>on! Inadvertent Local Variables Forgetting the ampersand (&) creates a call-by-value parameter The value of the variable will not be changed The formal parameter is a local variable that has no effect outside the function Hard error to find it looks right! 10/25/16 Matni, CS16, Fa16 15

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17 Using Procedural Abstrac>on Functions should be designed so they can be used as black boxes To use a function, the declaration and comment should be sufficient Programmer should not need to know the details of the function to use it 10/25/16 Matni, CS16, Fa16 17

18 Func>ons Calling Func>ons A function body may contain a call to another function The called function declaration must still appear before it is called Functions cannot be defined in the body of another function void order (int&, int&); void swap_values (int&, int&); int main () { order (a, b); return 0; } void order(int& n1, int& n2) { if (n1 > n2) swap_values(n1, n2); } void swap_values(int& n1, int& n2) { int temp = n2; n2 = n1; n1 = temp; } 10/25/16 Matni, CS16, Fa16 18

19 Pre- and Post- Condi>ons Concepts of pre-condition and post-condition Pre-condition States what is assumed to be true when the function is called Function should not be used unless the precondition holds Post-condition Describes the effect of the function call Tells what will be true after the function is executed (when the precondition holds) If the function returns a value, that value is described Changes to call-by-reference parameters are described 10/25/16 Matni, CS16, Fa16 19

20 swap_values revisited Using preconditions and postconditions the declaration of swap_values becomes: void swap_values(int& n1, int& n2); //Precondition: variable1 and variable2 have // been given values // Postcondition: The values of variable1 and // variable2 have been interchanged 10/25/16 Matni, CS16, Fa16 20

21 Func>on Celsius Preconditions and post-conditions make the declaration for celsius(): double celsius(double farenheit); //Precondition: fahrenheit is a temperature // expressed in degrees Fahrenheit //Postcondition: Returns the equivalent temperature // expressed in degrees Celsius 10/25/16 Matni, CS16, Fa16 21

22 Why use Pre- and Post- condi>ons? Pre-conditions and post-conditions should be the first step in designing a function Specify what a function should do: Always specify what a function should do before designing how the function will do it This minimizes design errors and time wasted writing code that doesn t match the task at hand 10/25/16 Matni, CS16, Fa16 22

23 Case Study: Supermarket Pricing Problem definition Determine the retail price of an item, given a suitable input 5% markup if item should sell in a week 10% markup if item expected to take more than a week 5% for up to 7 days, changes to 10% at 8 days Primary analysis Input: The wholesale price and the estimate of days until item sells (turnover) Output: The retail price of the item What subtasks do you think we need here? 10/25/16 Matni, CS16, Fa16 23

24 Supermarket Pricing: Problem Analysis Three main subtasks: Input the data Compute the retail price of the item Output the results Each task can be implemented with a function Note the use of call-by-value and call-by-reference parameters in the following function declarations (next slide) 10/25/16 Matni, CS16, Fa16 24

25 Supermarket Pricing: Func>on get_input void get_input(double& cost, int& turnover); //Precondition: User is ready to enter values correctly. //Postcondition: The value of cost has been set to // the wholesale cost of one item. // // The value of turnover has been // set to the expected number of // days until the item is sold. 10/25/16 Matni, CS16, Fa16 25

26 Supermarket Pricing: Func>on price double price(double cost, int turnover); //Precondition: cost is the wholesale cost of one item // turnover is the expected // number of days until the item is sold. // //Postcondition: returns the retail price of the item 10/25/16 Matni, CS16, Fa16 26

27 Supermarket Pricing: Func>on give_output void give_output(double cost, int turnover, double price); //Precondition: cost is the wholesale cost of one item; // turnover is the expected time until sale // of the item; // price is the retail price of the item. // //Postcondition: The values of cost, turnover, and price // are written to the screen. 10/25/16 Matni, CS16, Fa16 27

28 Supermarket Pricing: main() func>on With the functions declared, we can write the main function: int main() { double wholesale_cost, retail_price; int shelf_time; get_input(wholesale_cost, shelf_time); retail_price = price(wholesale_cost, shelf_time); give_output(wholesale_cost, shelf_time, retail_price); return 0; } 10/25/16 Matni, CS16, Fa16 28

29 Supermarket Pricing: Algorithm Design of price func>on Implementations of get_input and give_output are straightforward, so we ll concentrate on the price function pseudocode for the price function: If turnover <= 7 days then return (cost + 5% of cost); else return (cost + 10% of cost); 10/25/16 Matni, CS16, Fa16 29

30 Supermarket Pricing: Constants for the price func>on The mark-up values and the threshold number won t change, so we can represent them with constants: const double LOW_MARKUP = 0.05; // 5% const double HIGH_MARKUP = 0.10; // 10% const int THRESHOLD = 7; // At 8 days use HIGH_MARKUP 10/25/16 Matni, CS16, Fa16 30

31 Supermarket Pricing: Coding the price func>on The body of the price function { if (turnover <= THRESHOLD) return ( cost + (LOW_MARKUP * cost) ) ; else return ( cost + ( HIGH_MARKUP * cost) ) ; } See the complete program in the textbook, Chapter 5, pages /25/16 Matni, CS16, Fa16 31

32 Supermarket Pricing : Program Tes>ng What are some testing strategies that we can use? TEST COVERAGE: Covering all possible conditions of inputs and checking the outputs against expected results. Use data that tests both the high and low markup cases Test boundary conditions, where the program is expected to change behavior or make a choice In function price, 7 days is a boundary condition Test for exactly 7 days as well as one day more and one day less 10/25/16 Matni, CS16, Fa16 32

33 10/25/16 Matni, CS16, Fa16 33

34 Coun>ng Numbers in Different Bases We normally count in 10s Base 10: decimal numbers Number symbols are 0 thru 9 Computers count in 2s Base 2: binary numbers Number symbols are 0 and 1 Represented with 1 bit (2 1 = 2) Other convenient bases in computer architecture: Base 8: octal numbers Number symbols are 0 thru 7 Represented with 3 bits (2 3 = 8) Base 16: hexadecimal numbers Number symbols are 0 thru F A = 10, B = 11, C = 12, D = 13, E = 14, F = 15 Represented with 4 bits (2 4 = 16) Why are 4 bit representa0ons convenient??? 10/25/16 Matni, CS16, Fa16 34

35 Natural Numbers Coun>ng 642 as is coun>ng in TENS (aka BASE 10) There are 6 HUNDREDS There are 4 TENS There are 2 ONES = nd posi>on 1 st posi>on 0 th posi>on 10/25/16 Matni, CS16, Fa16 35

36 Posi>onal Nota>on in Decimal Continuing with our example 642 in base 10 positional notation is: 6 x x x 10 0 = 6 x 100 = 600 = 4 x 10 = 40 = 2 x 1 = 2 = 642 in base 10 10/25/16 Matni, CS16, Fa16 36

37 Posi>onal Nota>on What if 642 is expressed in the base of 13? 6 x 13 2 = 6 x 169 = x 13 1 = 4 x 13 = x 13º = 2 x 1 = 2 = 1068 in base 10 Anything à DEC So, 642 in base 13 is equivalent to 1068 in base 10 10/25/16 Matni, CS16, Fa16 37

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39 X 10/25/16 Matni, CS16, Fa16 39

40 2 10/25/16 Matni, CS16, Fa16 40

41 Posi>onal Nota>on in Binary in base 2 positional notation is: 1 x 2 4 = 1 x 16 = x 2 3 = 1 x 8 = x 2 2 = 1 x 4 = x 2 1 = 1 x 2 = x 2 0 = 1 x 1 = 1 So, 1011 in base 2 is = 27 in base 10 10/25/16 Matni, CS16, Fa16 41

42 Conver>ng Binary to Decimal Q: What is the decimal equivalent of the binary number ? A: Look for the posieon of the digits in the number. This one has 7 digits, therefore posieons 0 thru x 2 6 = 1 x 64 = x 2 5 = 1 x 32 = x 2 4 = 0 x 16 = x 2 3 = 1 x 8 = x 2 2 = 1 x 4 = x 2 1 = 1 x 2 = x 2º = 0 x 1 = 0 = 110 in base 10 10/25/16 Matni, CS16, Fa16 42

43 Conver>ng Binary to Octal and Hexadecimal (or any base that s a power of 2) Binary is 1 bit Octal is Hexadecimal is 3 bits 4 bits Use the group the bits technique Always start from the least significant digit Group every 3 bits together for bin à oct Group every 4 bits together for bin à hex 10/25/16 Matni, CS16, Fa16 43

44 Conver>ng Binary to Octal and Hexadecimal Take the example: to octal: in octal to hexadecimal: A6 in hexadecimal 10/25/16 Matni, CS16, Fa16 44

45 Conver>ng Decimal to Other Bases Algorithm for conver>ng number in base 10 to other bases While (the quotient is not zero) 1. Divide the decimal number by the new base 2. Make the remainder the next digit to the left in the answer 3. Replace the original decimal number with the quotient 4. Repeat until your quotient is zero Example: What is 98 (base 10) in base 8? 98 / 8 = 12 R 2 12 / 8 = 1 R 4 1 / 8 = 0 R /25/16 Matni, CS16, Fa16 45

46 Conver>ng Decimal into Binary Convert 54 (base 10) into binary and hex: 54 / 2 = 27 R 0 27 / 2 = 13 R 1 13 / 2 = 6 R 1 6 / 2 = 3 R 0 3 / 2 = 1 R 1 1 / 2 = 0 R 1 54 (decimal) = (binary) = 36 (hex) Sanity check: = = 54 10/25/16 Matni, CS16, Fa16 46

47 TO DOs Homework #9 due Thursday 10/27 Lab #5 Due Friday, 10/28, at noon 10/25/16 Matni, CS16, Fa16 47

48 10/25/16 Matni, CS16, Fa16 48